Low Alloy Steels with Enhanced Toughness and Fatigue Strength at High Hardness

ABSTRACT

Methods of forming low alloy steels and steels produced by such methods are provided. Various alloy additions and elements, as well as heating and tempering times and method steps are provided herein. Methods and materials of the present disclosure provide for enhanced fatigue at high hardness as compared with more brittle conventional steels.

This U.S. Non-Provisional patent application claims the benefit of priority from U.S. Provisional Patent Application Ser. No. 62/526,704, filed Jun. 29, 2017, the entire disclosure of which is hereby incorporated by reference.

FIELD

The present disclosure relates generally to methods of forming alloy steels and to devices and materials formed by such methods. More specifically, embodiments of the present disclosure provide for methods of forming steels with enhanced structural properties.

SUMMARY

There has been a long-felt but unmet need to provide alloy steels with enhanced structural properties and to provide lightweight materials with desired strength characteristics and material properties.

Embodiments of the present disclosure contemplate steel alloy materials and methods of making the same. In various embodiments, induction-hardenable ferrous metals are provided that contain at least one of the following alloy elements: carbon, manganese, tungsten, nickel, molybdenum, chromium, phosphorous, niobium, vanadium, aluminum, titanium, nitrogen, and/or boron. In some embodiments, induction-hardenable steels are provided that contain about 0.50 to 0.75 percent carbon by weight, about 0.2 to 1.7 percent manganese by weight, about 0.1 to 1.0 percent tungsten by weight, about 0.1 to 3 percent nickel by weight, up to about 0.5 percent molybdenum by weight, up to about 1.5 percent chromium by weight, up to 0.02 percent phosphorous by weight, up to about 0.025 percent niobium by weight, up to about 0.2 percent vanadium by weight, up to about 0.1 percent aluminum by weight, up to about 0.05 percent titanium by weight, up to about 0.02 percent nitrogen by weight, about 0.0005 to 0.003 percent boron by weight, and the balance iron with the usual impurities in conventional amounts.

In some embodiments, steel alloys are provided comprising austenite grains smaller than about 20 μm in diameter. The steel alloys are transformed to martensite with some retained austenite by rapid heating and quenching at rates that are attainable with induction heating and quenching. The small austenite grain size of embodiments of the present disclosure, along with control of alloy additions at austenite grain boundaries enables production of materials and components with enhanced ductility, notched fracture and fatigue strengths at increased carbon and hardness levels. The provision of such materials in accordance with methods and systems of the present disclosure results in the production of lighter and stronger components.

Devices and methods of the present disclosure contemplate controlling an amount of manganese, nickel, tungsten, molybdenum, chromium, phosphorous, niobium, vanadium, aluminum, titanium, nitrogen, and/or boron and obtaining relatively small austenite grain sizes over a workable range of induction heating rates and peak temperatures for the manufacture of parts with high hardness. The small austenite grain sizes of embodiments of the present disclosure, and favorable effects of elemental additions provide for ductile trans-granular fracture at hardness in excess of 595 HV (55 HRC) and up to 835 HV (65 HRC). By reducing or preventing inter-granular fracture, lighter, stronger parts and materials are achieved by embodiments of the present disclosure. Such parts and materials may be provided, for example, for use with highly stressed automotive driveline parts including, but not limited to, drive-shafts, axles and gears.

In one embodiment, a method of forming a steel alloy is provided comprising the steps of: providing a carbon steel; adding an alloy addition wherein the one alloy addition comprises not more than approximately 2.0 weight percent of the combined carbon steel and the alloy addition; austenitizing the carbon steel and the at least one alloy addition for at least approximately 1,000 seconds at a temperature of at least 900 degrees Celsius; quenching the carbon steel and the at least one alloy addition; tempering the carbon steel and the at least one alloy addition at a temperature of not more than approximately 250 degrees Celsius; performing a second austenitizing step; and further quenching and tempering the carbon steel and the at least one alloy addition at a temperature of not more than approximately 250 degrees Celsius.

In one embodiment, a steel alloy is provided that is formed by a method comprising the steps of: providing a carbon steel; adding an alloy addition wherein the one alloy addition comprises not more than approximately 2.0 weight percent of the combined carbon steel and the alloy addition; austenitizing the carbon steel and the at least one alloy addition for at least approximately 1,000 seconds at a temperature of at least 900 degrees Celsius; quenching the carbon steel and the at least one alloy addition; tempering the carbon steel and the at least one alloy addition at a temperature of not more than approximately 250 degrees Celsius; performing a second austenitizing step; and further quenching and tempering the carbon steel and the at least one alloy addition at a temperature of not more than approximately 250 degrees Celsius. The resultant steel alloy comprises a hardness of at least approximately 650 H_(Veq) and a torsional fatigue strength of at least approximately 650×10⁵ MPa.

The Summary is neither intended nor should it be construed as being representative of the full extent and scope of the present disclosure. The present disclosure is set forth in various levels of detail in the Summary as well as in the attached drawings and the Detailed Description and no limitation as to the scope of the present disclosure is intended by either the inclusion or non-inclusion of elements, components, etc. in this Summary. Additional aspects of the present disclosure will become more readily apparent from the Detailed Description, particularly when taken together with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the disclosure and together with the general description of the disclosure given above and the detailed description of the drawings given below, serve to explain the principles of these inventions.

It should be understood that the drawings are not necessarily to scale. In certain instances, details that are not necessary for an understanding of the disclosure or that render other details difficult to perceive may have been omitted. It should be understood, of course, that the disclosure is not necessarily limited to the particular embodiments illustrated herein.

FIG. 1 is a graph depicting material fracture as a function of hardness and torsional fatigue strength.

FIG. 2 depicts the effects of alloy additions on grain size.

FIG. 3 illustrates grain size as various elements are varied according to embodiments of the present disclosure.

FIG. 4 illustrates grain boundary sizes for various alloy steels processed at certain temperatures.

FIG. 5 compares peak stress and hardness for various materials.

FIG. 6 illustrates the effects of phosphorous and carbon on grain boundaries.

FIG. 7 compares fatigue limits of various materials as a function of grain size.

FIG. 8 is a stress-strain plot illustration features and materials of the present disclosure.

FIG. 9 is a plot showing various alloy elements and their effects on grain boundary strength.

FIG. 10 is a matrix of various steels and alloy additions contemplated by embodiments of the present disclosure.

DETAILED DESCRIPTION

FIG. 1 is a graph depicting material fracture as a function of hardness and torsional fatigue strength. Hardness and fatigue strength increase generally linearly in many carbon steels, with ductile fracture failures comprising the upper limits. Grain boundary fractures (illustrated in solid points) in existing steels represent a limit for the carbon steels shown in FIG. 1, wherein there is no substantial increase in fatigue strength even as hardness of the steel(s) is increased. Embodiments of the present disclosure contemplate methods of forming steels with increased fatigue strength over the materials illustrated in FIG. 1.

It is known that alloy elements tend to segregate to prior austenite grain boundaries (“PAGB”) during conventional austenitizing and tempering methods. Embodiments of the present disclosure contemplate the provision of shorter austenitizing times to provide different effects on PAGB segregation and mobility and to obtain enhanced material characteristics. In various embodiments, shorter austenitizing times are achieved through induction hardening methods. Embodiments of the present disclosure further contemplate the provision of various alloy and impurity elements to alter prior austenite grain size (“PAGS”) and affect grain boundary strength during short austenitizing times. Certain embodiments contemplate the provision of various alloy additions with 0.55% carbon steels. Molybdenum, phosphorous, chromium, nickel, manganese, silicon, antimony, tin, and arsenic are elements known to segregate to PAGB during temper embrittlement at about 450-600° C. Carbon, phosphorous, molybdenum, manganese, silicon, nickel and cobalt are known to segregate to PAGB during quench embrittlement. Low carbon alloys such as boron, manganese, carbon, chromium, molybdenum, and nickel, for example segregate to PAGB in austenitic steels. These alloy additions are contemplated for use with embodiments and methods of the present disclosure.

FIG. 2 illustrates the effects of various alloy additions on PAGS. As shown, the addition of various elements to low-carbon steels provides a significant impact on the resulting PAGS at least when the material is austenitized as provided in FIG. 2. FIG. 2 depicts a plurality of images (labeled a-f) depicting the PAGS of a Cr—Mo steel. Each sample is austenitized at sequentially increasing temperatures between approximately 900 and 1150 C. The resultant grain size is shown in FIG. 2, and wherein the grain size increases (from a to f) with increased temperature of the austenitization step.

FIG. 3 illustrates the effect of various alloy additions on PAGS at 900° C. and 1000° C. induction hardening temperatures. FIG. 3 also provides PAGS as a function of molybdenum content. As shown in FIG. 3, PAGS can be significantly impacted by element additions (type and % mass), and processing temperature.

FIG. 4 shows PAGS as a function of heating rate, maximum heating temperature, and time the material is held at a maximum heating point (and prior to the start of cooling). As shown, PAGS decreases as heating rate and time held at a maximum temperature increase. PAGS increases as maximum heating temperature increases.

At relatively long austenitizing times, elements segregate into PAGB at varying ratios. Carbon and substitutional alloys affect PAGB at these relatively long austenitzing times. Applicant has found that substitutional elements can affect PAGS with relatively short austenitizing times as well.

FIG. 5 illustrates the relationship between peak stress and hardness of various steels. As shown in FIG. 5, hardness and peak stress generally increase proportionally until a certain hardness is reached, whereafter a decrease in peak stress is shown to correspond with an increase in hardness above a certain value (about 50-55 HRC). The embrittlement of steel as a limiting factor on peak stress is illustrated in FIG. 5. FIG. 6 illustrates the effects of phosphorous and carbon on grain boundaries.

FIG. 7 illustrates the general increase in fatigue limit with respect to both grain size and phosphourous content. FIG. 8 provides a stress-strain plot of different steels. As shown, developed steel with between about 0.4% and 0.5% carbon comprises a relatively higher yield stress and lower ductility than various fine-grained steels.

FIG. 9 is a plot of showing various alloy elements and their effects on grain boundary strength. Quench embrittlement is generally characterized by grain boundary carbide precipitation, wherein long austenitizing times for carburizing or conventional through-hardening enhances GB segregation. Grain size refinement changes fracture initiation mode (typically from intergranular to surface ground boundary oxidation) after carburizing. Applicant has found that limiting austenitizing times and temperatures can result in finer grain sizes and increased ductility in a steel material.

In various embodiments of the present disclosure, alloy additions were provided in combination with a steel comprising 0.55% carbon, 0.008% nitrogen, and 0.015% sulfur. The alloy additions include at least one of manganese, silicon, chromium, nickel, molybdenum, tungsten, niobium, vanadium, titanium, aluminum, phosphourous, and boron. A composed alloy is treated by austenitizing the material at 900° C. and 1150° C., quenching the alloy, and tempering the alloy at approximately 175° C. In various embodiments, the tempered materials are then processed on a GLEEBLE™ device with austenitizing times of 2 seconds, 10 seconds, or 1000 seconds at 850° C., 950° C., and 1050° C., respectively, and quenched and tempered at approximately 175° C.

Various steels as shown in FIG. 10 are heat-treated on a GLEEBLE™ 3500. In some embodiments, a blank of steel is provided in the GLEEBLE and heated at 50° C./s to 850° C., 950° C., and/or 1050° C. in 2 seconds, 10 seconds, or 1000 seconds. The blank is then quenched at 140° C/s. Blanks are then subjected to a notched bend test. A fractured surface was formed and hardness testing and optical and SEM characterization were performed at or proximal to the fracture surface.

Methods according to embodiments of the present disclosure limit an inter-granular crack initiation in alloy steels. In various embodiments of the present disclosure, alloys are provided comprising enhanced fatigue strength at higher harnesses.

While various embodiments of the disclosed device have been described in detail, it is apparent that modifications and alterations of those embodiments will occur to those skilled in the art. However, it is to be expressly understood that such modifications and alterations are within the scope and spirit of the present disclosure, as set forth in the following claims. Further, the invention(s) described herein are capable of other embodiments and of being practiced or of being carried out in various ways. In addition, it is to be understood that the phraseology and terminology used herein is for the purposes of description and should not be regarded as limiting. The use of “including,” “comprising,” or “adding” and variations thereof herein are meant to encompass the items listed thereafter and equivalents thereof, as well as, additional items.

The foregoing discussion has been presented for purposes of illustration and description. The foregoing is not intended to limit the disclosure to the form or forms disclosed herein. In the foregoing description for example, various features of the disclosure have been identified. It should be appreciated that these features may be combined together into a single embodiment or in various other combinations as appropriate. The dimensions of the component pieces may also vary, yet still be within the scope of the disclosure. Moreover, though the description has included description of one or more embodiments and certain variations and modifications, other variations and modifications are within the scope of the disclosure, e.g. as may be within the skill and knowledge of those in the art, after understanding the present disclosure. It is intended to obtain rights which include alternative embodiments to the extent permitted, including alternate, interchangeable and/or equivalent structures, functions, ranges or steps to those claimed, whether or not such alternate, interchangeable and/or equivalent structures, functions, ranges or steps are disclosed herein, and without intending to publicly dedicate any patentable subject matter.

The present disclosure, in various embodiments, includes components, methods, processes, systems and/or apparatus substantially as depicted and described herein, including various embodiments, subcombinations, and subsets thereof. Those of skill in the art will understand how to make and use the devices of the disclosure after understanding the present disclosure. The present disclosure, in various embodiments, includes providing devices and processes in the absence of items not depicted and/or described herein or in various embodiments hereof, including in the absence of such items as may have been used in previous devices or processes, e.g., for improving performance, achieving ease and/or reducing cost of implementation. Rather, as the following claims reflect, inventive aspects lie in less than all features of any single foregoing disclosed embodiment. 

What is claimed is:
 1. A method of forming a steel alloy, comprising: providing a carbon steel with between approximately 0.40 and approximately 0.60 carbon by weight percent; adding at least one of manganese, nickel, molybdenum, and tungsten as an alloy addition wherein the at least one alloy addition comprises not more than approximately 2.2 weight percent of the combined carbon steel and the alloy addition; austenitizing the carbon steel and the at least one alloy addition for between 1500 seconds and 2000 seconds at 900 degrees Celsius and at 1150 degrees Celsius; quenching the carbon steel and the at least one alloy addition in the presence of helium; tempering the carbon steel and the at least one alloy addition at a temperature between approximately 150 degrees Celsius and 250 degrees Celsius; performing a further austenitizing step comprising at least one of induction heating and direct-resistance heating; and quenching and tempering the carbon steel and the at least one alloy addition at between approximately 150 degrees Celsius and 250 degrees Celsius.
 2. The method of claim 1, wherein the at least one of induction heating and direct-resistance heating comprises providing the carbon steel and the at least one alloy addition at 850 degrees Celsius for 2 seconds, at 950 degrees Celsius for 10 seconds, and 1050 degrees Celsius for 1000 seconds.
 3. The method of claim 1, wherein the step of quenching and tempering the carbon steel and the at least one alloy addition is performed at a temperature of approximately 200 degrees Celsius.
 4. The method of claim 1, wherein at least one of the austenitizing steps is performed with a GLEEBLE™ direct resistance heating system.
 5. A method of forming a steel alloy, comprising: providing a carbon steel; adding an alloy addition wherein the one alloy addition comprises not more than approximately 2.0 weight percent of the combined carbon steel and the alloy addition; austenitizing the carbon steel and the at least one alloy addition for at least approximately 1,000 seconds at a temperature of at least 900 degrees Celsius; quenching the carbon steel and the at least one alloy addition; tempering the carbon steel and the at least one alloy addition at a temperature of not more than approximately 250 degrees Celsius; performing a second austenitizing step; and further quenching and tempering the carbon steel and the at least one alloy addition at a temperature of not more than approximately 250 degrees Celsius.
 6. The method of claim 5, wherein the carbon steel comprises between approximately 0.40 and approximately 0.60 carbon by weight percent.
 7. The method of claim 5, wherein the alloy addition comprises at least one of manganese, nickel, molybdenum, and tungsten.
 8. The method of claim 5, wherein at least one of the austenitizing steps is performed with a GLEEBLE™ direct resistance heating system.
 9. The method of claim 5, wherein the carbon steel and the at least one alloy addition are austenitized for between approximately 1500 seconds and 2000 seconds.
 10. The method of claim 5, wherein the step of quenching is performed in the presence of helium.
 11. The method of claim 5, wherein the second austenitizing step comprises at least one of induction heating and direct-resistance heating. 